1. Field of the Invention
This invention relates to a radial tire for trucks and buses, and more particularly to a radial tire for trucks and buses with textile cords in its carcass ply layer and steel cords in its belt layer.
2. Prior Art
It has been well known that the textile cords used in tires as a reinforcing element will be extended to an extent depending upon its material, treatment conditions, number of twists, etc., under temporary or permanent tension. In the tires for trucks and buses that have the textile cords in their carcass ply layer with relatively high air pressure therein, the carcass ply cords tend to stretch out. In case steel cords are used for the belt layer, the tires will have a very limited extension in the outward direction. This will cause the tires to expand in the lateral direction absorbing the extension of the carcass ply cords. As a result, the shearing strain in the shoulder portions which are the jointing area of the tread and the side-wall, will increase. In addition, the shoulder portions have the ends of the steel cords of the belt layer, and this has been a major cause for separation. Due to these structural conditions, the shoulder portions are the weakest spot for breakage in the tires.
A variety of experimental tests have been performed to seek out the best conditions for improving the durability of the shoulder portions. The experimental tests have revealed that the durability of the tire depends to a large degree on the width of a tire when inflated with the air pressure of use; more particularly such tire width is determined in proportion to a tread width which does not lower the wear resistance of the tread or to the change of such width of a tire before and after inflation of the tire.